There are a number of benefits to being able to accurately estimate the velocity of a sea-faring vessel. One is that an accurate estimation of velocity results in improved estimates of ship's position when using dead-reckoning navigation, such as for submerged vehicles (e.g., submarines, AUVs, UUVs, etc.). This is particularly important when a submerged vehicle is unable to obtain Global Positioning System (GPS) fix information, such as when the vessel must maintain covert status. Another benefit of accurate velocity estimation is that it improves the accuracy of certain on-board missile-delivery systems that require an initial velocity as part of an initialization data set provided to the missile before launch (i.e., for the reduction of missile Circular Error Probable (CEP)).
Correlation SONAR technology is widely-used to estimate velocity. See, for example, U.S. Pat. No. 4,244,026 to Dickey and U.S. Pat. No. 5,315,562 to Bradley et al. These systems typically include a sonar source and a receiver array (i.e., hydrophone array). The positions of the receivers within the array are fixed and known. The SONAR source directs a series of acoustic pulses towards the ocean floor, and the receivers detect echoes of those pulses. The velocity of the vessel is then calculated based upon the best correlated pair of receivers within the array and/or the best correlated pulses detected by the receiver array.
Correlation SONAR systems can be water or ground referenced, and spatial-or temporal-based. A water-referenced correlation SONAR uses echoes reflected from the water beneath a vessel, whereas a ground-referenced correlation SONAR uses echoes reflected from the ocean bottom. A correlation SONAR can also be both ground and water based in the sense of having both ground- and water-referenced modes of operation.
Correlation SONARS rely on selecting a best or maximum correlation between hydrophones and/or pulses, for an estimation of velocity. Maximum correlation occurs when the ray path of an initial SONAR transmission (from the transmitter to the ocean floor, etc., and back to a receiver) of a first detected pulse is equal to the ray path of a second SONAR transmission.
Spatial-correlation SONAR estimates the velocity of a vessel by transmitting two or more pulses towards the ocean bottom, detecting echoes of the pulses on a planar two-dimensional array of hydrophones, determining which two hydrophones in the array correlate the best, and dividing the distance between those hydrophones by twice the time differential between the pulses. The time differential between the detected pulses for which maximum correlation occurs is referred to as the “optimal-correlation time,” CTo. In some cases, no two hydrophones will have a spacing that results in a maximum correlation. For example, peak correlation may occur between two hydrophone locations. In this case, an interpolation scheme is used as a part of the velocity estimation. Interpolation, however, reduces the accuracy of the velocity estimate.
Temporal-correlation SONAR also estimates the velocity of a vessel by transmitting two or more pulses toward the ocean bottom and detecting echoes of the pulses at a hydrophone array. For a given pair of hydrophones, the temporal system determines which two pulses correlate the best, and calculates velocity by dividing the fixed distance between the hydrophones by twice the time differential between the two correlated pulses.
Velocity estimates from correlation SONAR are subject to a variety of different random errors and bias errors. To the extent that these types of errors can be reduced, the accuracy of the velocity estimates will improve. Correlation SONARS also have integrity issues in which serious performance degradation can occur in the event that there is an undetected failure in a hydrophone or hydrophone channel and the SONAR uses the faulty channel data for its velocity solution. The phrase “hydrophone channel” means the hydrophone itself, as well as the connectors and cabling to channel electronics, the electronics, and associated data-processing components.